Roll your own Coil for tuners and traps (Part 1)
Author: Frederick R. Vobbe, W8HDU
November 8, 2008
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Diagram of a coilCoils are used in a variety of different methods to match, tune, and resonate circuits. Some coils are as small as 32g wire around a 2mm form. Other coils are large, and use copper pipe in place of wire.

I remember making one coil that was 16" long, and consisted of 1/2" soft copper water line wound around a 20" form. This was used in an AM directional antenna array. I've also made coils from #14 solid copper wire, wound around drill bits or broom handles to form them to a shape.

As the frequency increases, most of the time the size of the coil will get smaller. But this is not always the case. Careful calculation can optomize the circuit's efficiency and Q.

There are several variables.

  • L = inductance in microhenrys.
  • d = coil diameter in inches, which is from the center of the wire center to the opposite side center.
    Note: Not inside diameter (ID), or outside diameter (OD).
  • l = coil length in inches. For my Canadian friends, 1 inches = 25.4 mm.
  • n = number of turns of the coil.

Usually you will need to calculate for one of two values, the number of turns, or the value in microhenrys. The formulas are as follows.

Number of turns formula Value in microhenries formula

The coil can be compact with no space between the windings, or you can pull them apart. Typically when you pull them apart you get the best Q. In most HF work this can be .250", or 1/2 to 1 times the diameter of the wire you're using.

When making coils be sure to consider the environment, and the rating. If the coil is inside equipment it may not need any protection. If the coil is outside, (perhaps in a tuning house), precaution should be taken for variations in temperture, physical fatigue, and safety. As far as the rating goes, some circuits can develop amazing amounts of currents and power. In an application such as an amplifier, or high power RF, stay away from small size wires.

Coils can be fixed values or variable. A fixed value looks like this;

Fixed value coil

While a variable can have one, or two tap points to allow for changing of values, like these;

Variable coil Multi-tap coil

A typical application for an RF coil would be in a tuner at the output of a transmission line at an antenna.

Let's assume we have a transmitter in our shack, and were feeding a vertical out in the backyard. We'll assume that the frequency is 1,850 KHz, and have 1,000 watts of power. The transmitter output is 50-ohm with no reactive value (50+j0), and the antenna is 40 ohms and has a reactive component of 100. (40+j100). We're using Belden RG-8 50-ohm transmission line. We would need a tuner at the base of the antenna to match the 50 ohm line to the antenna. It would look something like this;

Typical tuning system at the base of an antenna

The one thing to note on this is the HIGH voltages on components C2 and C3. These voltages are enought to kill, so you need to take precautions for both arcing of these devices, as well as safety. For example, suppose your operating a contest and your child or pet wanders into the area where these devices are locate?

Tapping a Coil

Gator clipsA good method for tapping coils is an alligator clip. You can easily tap a coil, yet change the position (value) by moving the clip. You should ALWAYS do this with the circuit NOT ENERGIZED. (See above).

These clips come in a variety of different styles and sizes. The only precaution to take when you use clips, (besides the obvious mentioned above), is;

  1. In mobile use the clips may become detatched from the coil.
  2. The clip may not have sufficient metal-to-metal contact to sustain high current contact on the coil.
  3. The clip may be too thick, and touch adjacent windings of the coil.
  4. After a while, the spring on the clip may fatigue, causing loss of connection or resistance.

Now that you know how to design your own coil, how do you make one?

to be continued

Frederick R. Vobbe, W8HDU

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